Topping A90

At only $100 more I’d definitely go for the A90D, but in the end it really matters what sound characteristics you’re looking for.  The A90s definitely skew to more neutral and highly detailed realm — if you’re looking for any warmth or tonal richness look elsewhere.  I have the Class-A Singxer SA-1 and it has that Class-A natural smoothness that works really well with my Hifiman Arya Stealths yet I still don’t feel like I’m missing any detail or air up top, and imaging/soundstage is likewise excellent.  I’d recommend it, but it lacks a remote that I assume you’d want if also using the A90 as a stereo preamp.  Here’s a good review of the A90D with an in-depth comparison to the A90 that you may find helpful. 
https://soundnews.net/amplifiers/headphone-amps/topping-a90-discrete-review-forging-ahead/

If you’re after a very neutral and highly detailed presentation it’s hard seeing going wrong with the A90D.  If you’re looking for a little more meat on the bones I’d look elsewhere.  Hope this helps, and best of luck. 

@soix Thanks for the response. That is a good review on both models. The A90 that I am planning on getting is a used but in very mint condition unit. According to the seller, the unit has about 100 hours of playing time just enough of break in time to justify it sound. 

Will be using mainly as a headphone amp as right now I have a Vali 2 which I think sound good.

What headphones are you using, and do you need a remote for using as a stereo preamp?  

@soix I am using a Tascam headphones I believe is the TH-200x something like that.

I have been using the A90 extensively and all can say is WOW. As a headphone amp I am impressed. It took my TASCAM headphones to whole new level. I still have not used it as a preamp but will soon be the next test. Will see how it goes against my Rotel RC-995.